Frequency assignment among antennas in a cellular communications network

ABSTRACT

A cellular telecommunications network includes spaced apart base stations. Each base station has three directional antennas for transmission and reception of signals to subscriber units. Eight frequency groups are assigned among all the antennas such that each frequency group is reused on average at three out of every eight base stations.

This is a continuation of International Patent Application No.PCT/GB96/00950, with an International Filing Date of Apr. 19, 1996.

The present invention relates to a method of assigning carrierfrequencies to base stations in a cellular radio telecommunicationsnetwork, and to a radio telecommunications network including suchfrequency assignments.

In known cellular radio systems, such as for mobile telephones, anetwork of base stations are provided each having antennas. Mobilesubscriber units have antennas which are necessarily omni-directional assubscribers often move around, both within cells and from one cell toanother. In consequence, a seven-cell frequency reuse pattern is common,as described, for example, in Cellular Radio Systems, Balston D MMacario RCV Editors, Artech House Inc, 1993, page 9 to 13.

The invention is defined in the claims to which reference should now bemade. Preferred features are laid out in the dependent claims.

The present invention preferably provides a cellular telecommunicationsnetwork including spaced apart base stations, each base stationcomprising three directional antennas for transmission and reception ofsignals to subscriber units, eight frequency groups being assigned amongthe antennas such that each frequency group is reused on average atthree out of every eight base stations.

The azimuthal look directions of the three antennas of a base stationare preferably substantially 120° from each other. The base stations arepreferably spaced so as to be equidistant from its six nearestneighboring stations, and to lie in alternating parallel first andsecond rows such that antennas in first rows point at 0°, 120° and 240°azimuthal angles, and antennas in second rows point at 60°, 180° and300° azimuthal angles. The azimuthal angles are preferably clockwise andwith north being 0°.

A preferred base station has an antenna with a look direction using thesame frequency group as an oppositely--directed look direction of anantenna of a neighboring base station in an adjacent row.

Frequency groups are preferably assigned according to any of thefollowing rules:

(a) a frequency group is not assigned to more than one antenna at a basestation,

(b) the same frequency groups are assigned to corresponding antennas atevery sixth base along a row,

(c) the same frequency groups are assigned to corresponding antennas atevery twenty-fourth base station along a column,

(d) the antennas in a row having the same look direction of 0° or 180°azimuth angle are assigned alternating frequency groups along the row,and the antennas in the row having other look directions are assignedother frequency groups such that the frequency groups are not reused inadjacent antenna coverage areas. In particular, frequency groups f₁, f₃,f₄ are used in base stations at column i, row n; frequency groups f₂,f₅, f₃ are used in column i+1, row n, frequency groups f₁, f₄, f₅ areused in column i+2, row n.

(e) column i+2, row n+2 has frequency groups corresponding with thosefor column i, row n where the frequency groups are selected as

    fi+2,n+2=fi,n‡3

where f is the frequency group at a sector of the base station at columni row n,

fi+2, n+2 is the frequency group at the corresponding sector of the basestation at column i row n,

‡ indicates a circular addition up to 8.

The antennas at base stations in the network are preferably 120°beamwidth directional antennas, but might be of lower beamwidth, such as60 to 85°.

The network can comprise a selection of base stations configured andhaving frequency groups assigned so as to avoid selected poorcommunication paths, specifically those having high co-channelinterference. To avoid selected poor communication paths, alternate rowsof base stations in the network can substantially use alternate signalpolarisations. Selected base stations can have polarisations differentto other members of their row so as to avoid selected poor communicationpaths.

The present invention is preferably for use with subscriber units havingdirectional antennas. Preferably the direction(s) and location(s) of theor each antenna of a subscriber unit are fixed.

The present invention also relates to corresponding methods of frequencygroup assignment.

A frequency group can be a frequency, a range of frequencies or aselection of frequencies.

A preferred embodiment of the present invention will now be described byway of example and with reference to the drawings in which:

FIG. 1 illustrates a single base station with three antennas;

FIG. 2 illustrates the allocation of frequency bands within the basestation network;

FIG. 3 illustrates some selected signal paths for which signal to noiseratios are deternined; and

FIG. 4 illustrates the base station network modified such that alternaterows of base stations transmit and receive different polarisation.

FREQUENCY ASSIGNMENT IN THE BASE STATION NETWORK

As shown schematically in FIG. 1, a base station has three directionalantennas, each directed 120° apart in the horizontal plane such that thecell or base site of each base station is divided into three sectors. Anetwork of these base stations is shown in FIG. 2. In this network, thebase stations are distributed such that each base station is equidistantfrom its nearest six neighbours. Considering the base stations are lyingin rows which are shown horizontally in FIG. 2, each base station has aneighbour on either side in the same row and two neighbours on each ofthe adjacent rows.

It will be seen from FIG. 2 that the antennas on alternate rows arealigned differently. For example, the antenna look directions forantennas at base stations on rows n, n+2, n+4. . . n+2i point North(0°), 120° and 240°. The antennas at base stations on rows n+1, n+3. . .n+(2i-1) point at 60°, 180° and 300°. The base stations on rows n+1,n+3, are offset in an east-west direction by half the separation betweenbase stations on a row.

Eight frequency groups are used and these are assigned as follows:

(i) On every row, the same frequency groups are assigned at thecorresponding antennas at every sixth base station, (i_(bs) =1, 7, 13,19 etc).

(ii) On every column, running north-south, the same frequency groups areassigned at the corresponding antennas at every twenty-fourth basestation, (i_(bs) =1, 25, 49, 73 etc).

(iii) The north facing sectors of base stations in row n are alternatelyassigned frequency groups denoted 1 and 2 Frequency group numbers arelabels for frequency groups, neighboring numbers, eg. 1 and 2, can, butneed not be, adjacent frequencies in a frequency range. The othersectors of the base stations on this row use frequency groups denoted 3,4 and 5 assigned such that a particular frequency group is not used formore than one antenna at a base station nor in both of two neighbouringbase station sectors.

As shown in FIG. 2, the base station at column i (row n) uses frequencygroups 1,3,4; the station at column i+1 (row n) uses frequency groups2,5,3; the base station at column i+2 (row n) uses frequencies 1,4,5;and so on.

As shown in FIG. 2, each base station and a neighbouring base station inan adjacent row have antennas with oppositely--directed look directionsusing the same frequency group.

The base stations along row n+1 has frequency groups 1 and 2 alternatelyassigned to the south facing antennas and two frequency groups selectedfrom groups 6,7 and 8 on their other antennas such that a particularfrequency group is not used for more than one antenna at a base stationnor in both of two neighbouring base station sectors.

The base stations in column i+2, row n+2 are configured to correspond tothose allocated for column i, row n where the frequency groups areselected as

    fi+2,n+2=fi.n‡3

where f is the frequency group at a sector or the base station at columni row n,

fi+2, n+2 is the frequency group at the corresponding sector of the basestation at column i row n,

‡ indicates a circular addition up to 8, i.e. 8‡1=1

7‡3=2

2‡3=5.

Thus if a frequency group f is allocated to an antenna having a bearingof 60° at a base station in row n+1 column i+0.5, frequency group f isalso assigned to the antenna with a 240° bearing at a base station inrow n+2 column i+1. Also, if a frequency group f is allocated to anantenna having a bearing of 300° at a base station in row n+1 columni+0.5, frequency group f is also assigned to the antenna with a 120°bearing at a base station in row n+2 column i-1.

Signal to Interference Analysis on the Preferred Network

To consider the signal to interference ratios possible in the basestation network, a simple analysis has been undertaken to determinesignal to interference ratios at likely "worst case " locations. FIG. 3shows a subset of the base station network which is shown in FIG. 2 inwhich particular bas stations A to O are identified. Circled pointswhich are denoted P to Z indicate where poor reception, i.e. low signalto noise ratios, are expected. The frequency group used in the varioussectors are shown in FIG. 3. The broken lines shown in FIG. 3 indicatesignal transmission paths selected for determination of signal to noiseratios.

Signal to interference ratio calculations were made assuming realisticsignal strength values and that the signal strength decreases as afunction of distance cubed, i.e. d³ where d is distance. The antennas atthe base stations were taken as having a beam width of 120° and afront-to-back ratio, ie. maximum attenuation due to antennadirectionality of 18 dB. The fixed subscriber unit antennas were assumedto be highly directional having a beam width of 10° and a front-to-backratio of 20 dB. The points P to Z were set to lie at the edge of thesector in which they are located and at a distance r from the nearestantenna where the separation of antennas is at least 2 r. The basestation serving the sector in which a point P to Z is located is denotedthe server. An interfering base station is an "interferer".

                  TABLE 1                                                         ______________________________________                                        Losses from Server at Sampled Points.                                                          Frequency                                                                              Angle offset at                                                                         Loss at base                              Point Serving Base                                                                             Group    base station/.sup.o                                                                     station/dB                                ______________________________________                                        P     B          5        30        1                                         Q     C          6        60        3                                         R     B          3        60        3                                         S     B          5         0        0                                         T     G          3        60        3                                         U     L          6        60        3                                         V     J          2        25        1                                         W     K          1        60        3                                         X     J          7        60        3                                         Y     J          7        60        3                                         Z     L          2        10        0                                         ______________________________________                                    

The server and the frequency group for each of the points P to Z isshown in Table 1. The azimuthal offset angle between the direction tothe point P to Z and the look direction of its serving antenna is alsoindicated together with the corresponding azimuthal loss at the servingantenna.

In determining signal to interference ratios, the effect of interferingbases on the signals are determined, assuming all transmitters are ofthe same power. The paths to points P to Z and, in each case, theneighbouring base which uses the same frequency groups and so interferesare shown as the first two columns of Table 2. The frequency groups areshown in the third column, and the path distances which interferingsignals must travel relative to the corresponding path distances fordesired signals are shown in the fourth column of Table 2.

                  TABLE 2                                                         ______________________________________                                        Interference and C/I in poorest locations                                                                                    Signal                                                        Angle           to                             Path Inter-        Dis-  Angle offset                                                                              Loss Loss inter-                         to   fering        tance offset                                                                              (subsc                                                                              base,                                                                              (sub)                                                                              ference                        point                                                                              Base   Freq.  ratio-(r)                                                                           (base)                                                                              unit) /dB  /dB  /dB                            ______________________________________                                        P    C      5      3     150   0     18   0    31                             Q    K      6      2.6   50    110   2    20   31                             Q    A      6      3.1   71    48    5    20   36                             Q    I      6      4.0   15    165   0    20   35                             R    D      3      4.1   16    74    0    20   35                             S    G      5      4.77  60    0     3    0     23*                           T    D      3      1     150   120   18   20   35                             U    A      6      8.1   60    0     3    0     27*                           U    C      6      5.1   35    24    20   1    39                             V    0      2      7.2   35    0     1    0     25*                           W    1      1      5.07  80    10    7    3     28*                           X    B      7      2.6   0     180   0    20   29                             Y     B     7      4.3   13    47    1    20   36                             Y    D      7      6.5   50    10    2    3     26*                           Z    O      2      6.0   51    0     2    0     25*                           ______________________________________                                    

The fifth column shows the offset angles between the look directions ofthe antennas intended to communicate with points P to Z and thedirection from which the interfering signal of the same frequency grouporiginates.

The sixth column shows the angular offset between the interfering signaland a subscriber unit at the respective point. Each subscriber unit hasa directional antenna pointing directly towards its serving antenna.

The losses due to the angular offsets are shown in the seventh andeighth columns. These predicted attenuations are more severe than islikely actually to occur in practice.

The signal to interference values evaluated are shown in the last columnof Table 2, from which it can be seen that the location with the worstsignal to noise ratio is point 5 which has a signal to interferenceratio of 23 dB. Other poor locations are points U,Y,W,V, and Z. Thesepoints are indicated with asterisks in Table 2.

Ways to Improve Communications in the Preferred Network

The signal to interference ratios can be improved by selecting differentantenna beamwidths at base stations and/or by making use of signalpolarisation. These are considered in turn below.

Antenna Beamwidths

Instead of using antennas with a beamwidth of 120°, 60° beamwidthantennas can be used. Taking the attenuation at an angular offset of 60°from the antenna look direction as 12 dB, the signal to noise ratio inthe worst case would be only 32 dB.

As using three 60° antennas at each base station might leave gaps in thearea covered, antennas could be used having a sensitivity at 60° angularoffset which is approximately the same as an omnidirectional antenna.The worst case signal to interference ratio could be improved by around3 dB using 85° antennas without affecting coverage.

Polarisation

An alternative method to increase signal to interference ratio is to usecross polarisation. As shown in FIG. 4, alternate rows are givenalternately horizontal and vertical polarisations. Horizontalpolarisation areas are shown shaded. This reduces interference alongpaths to points corresponding to S V Z as shown in FIG. 3 by about 10 dBbecause the worst interferer uses signals cross-polarised to those ofthe server. The interference to points W and Y is not changed as theserver and interferer use the same polarisation.

In setting up base station networks in restricted areas, particular poorsignal to interference paths can be avoided. For example, if a longnarrow area is to be covered, two rows of base stations could be used,the polarity being alternated along each row. The poor signal tointerference ratio along paths corresponding to points V and Z would notarise as these are caused by bases 3 rows distant. Scenarios S, W and Ywould be avoided because of the attenuating polarisation.

Another example is where the area is covered is relatively small suchthat few base stations are required. By selecting up to eight basestations as shown within the example planning area shown in FIG. 4, theworst signal to noise ratios are avoided by appropriate choice ofpolarisation and because the selected planning area avoids interferencesof scenarios U and Y. As shown in FIG. 4, within the example planningarea, two base stations use the alternative polarisation to the row ofstations in which they lie. These two base stations are shown withdiagonal line shading in FIG. 4.

By using cross polarisation, signal to interference improvements aremade. The worst case ratio is about 26 dB (path to point Y), and manyunacceptable paths, having say signal to interference ratios of lessthan 30 dB, are avoided. Many other planning areas within a largernational network besides the example shown in FIG. 4 could be selectedand used on the ground so as to avoid poor communications.

I claim:
 1. A cellular telecommunications network including spaced apartbase stations, each of said base stations comprising three directionalantennas for transmission and reception of signals to subscriber units,said network comprising eight frequency groups being assigned among theantennas such that each frequency group is reused on average at threeout of every eight base stations, the base stations being spaced so asto lie in alternating parallel first and second rows such that antennasin first rows point at 0°, 120°, and 240° azimuthal angles, and antennasin second rows point at 60°, 180°, and 300° azimuthal angles.
 2. Acellular telecommunications network according to claim 1, in whichazimuthal look directions of the three antennas of each base station aresubstantially 120° from each other.
 3. A cellular telecomniunicationsnetwork according to claim 1, in which the base stations are spaced soas to be equidistant from its six nearest neighbouring stations.
 4. Acellular telecommunications network according to claim 3, in which theazimuthal angles are clockwise and with north being 0°.
 5. A cellulartelecommunications network according to claim 1 in which a first basestation has an antenna with a look direction using the same frequencygroup as an oppositely--directed look direction of an antenna of aneighbouring base station in an adjacent row.
 6. A cellulartelecommunications network according to claim 1 in which frequencygroups are assigned according to rule that a frequency group is notassigned to more than one antenna at a base station.
 7. A cellulartelecommunications network according to claim 1 in which frequencygroups are assigned according to the rule that the same frequency groupsare assigned to corresponding antennas at every sixth base along a row.8. A cellular telecommunications network according to claim 1 in whichfrequency groups are assigned according to the rule that the samefrequency groups are assigned to corresponding antennas at everytwenty-fourth base station along a column.
 9. A cellulartelecommunications network according to claim 1 in which frequencygroups are assigned according to the rule that the antennas in a rowhaving the same look direction of 0° or 180° azimuth angle are assignedalternating frequency groups along the row, and the antennas in the rowhaving other look directions are assigned other frequency groups suchthat the frequency groups are not reused in adjacent antenna coverageareas.
 10. A cellular telecommunications network according to claim 9,in which frequency groups f₁, f₃, f₄ are used in a base station atcolumn i, row n; frequency groups f₂, f₅, f₃ are used in a base stationat column i+1, row n, frequency groups f₁, f₄, f₅ are used in a basestation at column i+2, row n.
 11. A cellular telecommunications networkaccording to claim 1 in which frequency groups are assigned according tothe rule that a base station at column i+2, row n+2 has frequency groupscorresponding with those for a base station at column i, row n where thefrequency groups are selected as

    fi+2,n+2=fi,n‡3

where f is the frequency group at a sector of the base station at columni row n, fi+2, n+2 is the frequency group at the corresponding sector ofthe base station at colurnn i row n, and ‡ indicates a circular additionup to
 8. 12. A cellular telecommunications network according to claim 1in which the antennas at base stations in the network are 120° beamwidthdirectional antennas.
 13. A cellular telecommunications networkaccording to any of claims 1 in which the antennas at base stations inthe network are lower than 120° beamwidth.
 14. A cellulartelecommunications network according to claim 13, in which the antennasare of a beamwidth in the range 60° to 85°.
 15. A cellulartelecommunications network according to claim 1, comprising a selectionof base stations have frequency groups assigned so as to avoid selectedpoor communication paths.
 16. A cellular telecommunications networkaccording to claim 15, in which to avoid selected poor communicationpaths, alternate rows of base stations in the network substantially usealternate signal polarisations.
 17. A cellular telecommunicationsnetwork according to claim 15 in which selected base stations havepolarisations different to other members of their row so as to avoidselected poor communication paths.
 18. A cellular telecommunicationsnetwork according to claim 1 in which the subscriber units havedirectional antennas.
 19. A cellular telecommunications networkaccording to claim 18, in which the direction(s) and location(s) of theor each antenna of a subscriber unit is fixed.
 20. A cellulartelecommunications network according to claim 1 in which a frequencygroup is a frequency, a range of frequencies or a selection offrequencies.
 21. A method of frequency group assignment in a cellulartelecommunications network including spaced apart base stations, eachbase station comprising three directional antennas for transmission andreception of signals to subscriber units, the base stations being spacedas to lie in alternating parallel first and second rows such thatantennas in first rows point at 0°, 120°, and 240° azimuthal angles, andantennas in second rows point at 60°, 180°, and 300° azimuthal angles,whereby said network comprising eight frequency groups are assignedamong the antennas such that each frequency group is reused on averageat three out of every eight base stations.
 22. A method of frequencygroup assignment according to claim 21, in which frequency groups areassigned according to the rule that a frequency group is not assigned tomore than one antenna at a base station.
 23. A method of frequencyassignment according to claim 21, in which frequency groups are assignedaccording to the rule that the same frequency groups are assigned tocorresponding antennas at every sixth base along a row.
 24. A method offrequency assignment according to claim 21, in which frequency groupsare assigned according to the rule that the same frequency groups areassigned to corresponding antennas at every twenty-fourth base stationalong a column.
 25. A method of frequency assignment according to claim21, in which frequency groups are assigned according to the rule thatthe antennas in a row having the same look direction of 0° or 180°azimuth angle are assigned alternating frequency groups along the row,and the antennas in the row having other look directions are assignedother frequency groups such that the frequency groups are not reused inadjacent antenna coverage areas.
 26. A method of frequency groupassignment according to claim 25, in which the frequency groups f₁, f₃,f₄ are used in base stations at column i, row n; frequency groups f₂,f₅, f₃, are used in column i+1, row n, frequency groups f₁, f₄, f₅, areused in column i+2, row n.
 27. A method of frequency assignmentaccording to claim 2, in which frequency groups are assigned accordingto the rule that a base station at column i+2, row n+2 has frequencygroups corresponding with those for a base station at column i, row nwhere the frequency groups are selected as

    fi+2,n+2=fi,n‡3

where f is the frequency group at a sector of the base station at columni row n, fi+2, n+2 is the frequency group at the corresponding sector ofthe base station at column i row n, ‡ indicates a circular addition upto 8.